Studies in progress are designed to elucidate the mechanism of action of cyclic AMP in the production of smooth muscle relaxation. We are attempting to purify and characterize the substrate of the cyclic AMP-dependent protein kinase found in tracheal smooth muscle. Anticipated future studies are designed to investigate the physiological importance of this enzyme and its substrates, specifically to link phosphorylation in vivo of these substrates to the actions of the Beta- sympathomimetic drugs as well as to correlate degree of phosphorylation in vivo with the tone of the smooth muscle.